The fixture is a drilling template made of machined steel that properly places the hole above the selector for the GI auto sear pin. A bolt is used to hold the fixture in place and the top of the fixture is aligned parallel with the top of the receiver to align sear pin hole. There is a washer that is used to allow the fixture to clear the "selector stops" on the receiver. There is still a small amount of milling that has to be done to allow for clearance for the sear in the receiver.
ANY & All NFA RULES APPLY TO THE USE OF A CLASS II DRILLING FIXTURE